The present invention relates to an aberration compensating apparatus for use with a recording and/or reproducing apparatus of an information recording medium such as an optical disc, and a driving method therefor.
There are optical discs such as a CD (Compact Disc) and a DVD (Digital Video Disc or Digital Versatile Disc) as well-known information recording media for optical recording and reproduction of information. Furthermore, the optical discs are of various types, for example, optical discs for reproduction only, write-once optical discs on which only additional recording can be done, and rewritable optical discs on which information can be erased and re-recorded.
Research and development are in progress for realizing high-density optical discs and optical pickups and information recording and/or reproducing apparatus (hereinafter referred to as recording/reproducing apparatus) applicable to the high-density optical discs. In addition, research and development are also pursued for realizing optical pickups and information recording/reproducing apparatus having the capability to be used for optical discs of different types.
A method of coping with the high-density discs by increasing a numerical aperture (NA) of an objective lens provided in the pickup apparatus has been considered. Another method is the use of a light beam having a shorter wavelength.
However, the aberration of the light beam caused by an optical disc is increased as the numerical aperture NA of the objective lens is increased or a light beam having a shorter wavelength is used. This makes it difficult to improve accuracy of the recording/reproduction performance of information.
For example, when an objective lens having a large numerical aperture is used, the amount of birefringence distribution, which depends on the incidence angle, is increased at the pupil surface of the optical disc, since the range of the incidence angle of the light beam to the optical disc is increased. This creates a problem of spherical aberration due to the birefringence becoming more influential. In addition, when using an objective lens having a large numerical aperture and a light beam having a shorter wavelength, the influence of coma aberration can not be negligible if the incident angle of the light beam to the normal direction (tilt angle) of the optical disc tilts at the time of recording or reproduction.
As described above, optical discs of different types, for example, CDs and DVDs, differ in structure such as substrate (i.e., transparent cover layer) thickness of the discs in recording density, and the like. Consequently, the influence of aberrations such as spherical aberration, coma aberration, or astigmatism differs according to the disc type, thus, making it difficult to develop a compatible optical pickup and an information recording/reproducing apparatus. In addition, the magnitudes of aberration are different even for optical discs of the same type, since the substrate thickness varies due to, for example, variations in the manufacturing process.
In order to reduce the effects of aberrations, a conventional pickup has been proposed which comprises a liquid crystal unit for aberration compensation. As such a liquid crystal unit, there is, for example, such a unit disclosed in the Japanese Patent Application Kokai H10-20263.
FIG. 1 is a schematic view of an example of the liquid crystal unit. The liquid crystal unit is composed so that a liquid crystal element C is held between transparent electrode layers A and B opposing each other. The orientation state of the liquid crystal element C can be changed by adjusting the voltage applied between the transparent electrode layers A and B. It is designed so that incident light entering on the side of one transparent electrode layer A (or B) exits on the side of the other transparent electrode layer B (or A) and is provided with a birefringence change corresponding to the orientation state of the liquid crystal element C as the light passes therethrough.
Furthermore, the transparent electrode layers A and B are formed in a divided manner, for example, divided into a plurality of transparent electrodes a1, a2, and a3, and b1, b2, and b3. The transparent electrodes a1, a2, and a3 are electrically separated from each other, and the transparent electrodes b1, b2, and b3 are also electrically separated from each other.
Consequently, because the liquid crystal element C can be adjusted to have a plurality of variously oriented states by applying a different voltage between the transparent electrodes opposing each other, for example, between the transparent electrodes a1 and b1, a2 and b2, and a3 and b3 respectively, incident light can be provided with birefringence changes simultaneously corresponding to each of the oriented states. Thus, an aberration such as spherical aberration or coma aberration occurring in the optical path can be compensated by suitably adjusting the plurality of oriented states of the liquid crystal unit. As was mentioned above, a difference in substrate thickness can produce various cases, for example, one where the aberration is larger on the peripheral portion than in the central part of the optical path, or one where the aberration is smaller on the peripheral portion.
The liquid crystal unit having concentrical electrodes as shown in FIG. 1 is discussed as an example and a detailed description will be given by referring to FIG. 2. FIG. 2 shows the phase difference caused by a liquid crystal element depending upon the applied voltage. For example, incident light is provided with a phase difference by making the electrodes b1-b3 equipotential, by setting the electrode a1 as a reference voltage V1 (for example, V1=2 V), and by applying the voltages to the electrodes a2 and a3 different from that of the electrode a1. In order to increase the phase difference along the outer direction against the central part of an optical path, voltages V2 and V3 applied to the electrodes a2 and a3 are increased (for example, V2=2.2 V and V3=2.4 V when V1=2 V). In order to decrease the phase difference along the outer radius direction, the voltages V2 and V3 applied to the electrodes a2 and a3 are decreased (for example, V4=1.8 V and V5=1.6 V when V1=2 V).
However, the conventional liquid crystal units are required to generate a large amount (range) of phase difference for increasing or decreasing the phase difference in the other areas against the reference compensatory area.
The present invention has been made to overcome the above-described problems, and it is an object of the present invention to provide a high-performance aberration compensating apparatus capable of decreasing the required amount of phase difference, and a method thereof.
To achieve the object, according to one aspect of the present invention, there is provided an aberration compensating apparatus for compensating an aberration occurring in a light beam, the light beam being applied to a recording medium and reflected by the recording medium through an optical path of an optical system, which comprises a liquid crystal unit including a first electrode layer having a plurality of divisional electrodes electrically separated from each other in the same plane and a second electrode layer, and a liquid crystal element provided between the first and second electrode layer which provides a light beam passing therethrough with a phase change when an electric field is applied; a detector for receiving the reflected light beam through the liquid crystal unit to generate a detection signal; a voltage generator for generating voltages to be applied to each of the plurality of divisional electrodes; and a controller for performing compensation control for the aberration by changing the applied voltages to each of the divisional electrodes with reference to an applied reference voltage to a predetermined divisional electrode of the first electrode layer, wherein the controller determines the applied reference voltage based on an amplitude change of the detection signal when the applied voltage to each of the plurality of the divisional electrodes are changed.
According to another aspect of the present invention, there is provided a method of compensating an aberration occurring in a light beam, the light beam being applied to a recording medium and reflected by the recording medium through an optical path of an optical system, which comprises the steps of providing a liquid crystal unit including a first electrode layer having a plurality of divisional electrodes electrically separated from each other in the same plane and a second electrode layer, and a liquid crystal element provided between the first and second electrode layer which provides a light beam passing therethrough with a phase change when an electric field is applied; receiving the reflected light beam through the liquid crystal unit to generate a detection signal; generating voltages to be applied to each of the plurality of divisional electrodes; and performing compensation control for the aberration by changing the applied voltages to each of the divisional electrodes with reference to an applied reference voltage to a predetermined divisional electrode of the first electrode layer, wherein the step of performing compensation control determines the applied reference voltage based on an amplitude change of the detection signal when the applied voltage to each of the plurality of the divisional electrodes are changed.